The field of the invention relates to air/fuel control systems. In one particular aspect of the invention, the field relates to monitoring emissions of an internal combustion engine while controlled under an air/fuel control system.
U.S. Pat. No. 5,259,189 discloses an engine air/fuel control system responsive to a feedback variable derived from an exhaust gas oxygen sensor positioned upstream of a catalytic converter. The catalytic converter is monitored by a hydrogen and/or carbon monoxide sensor positioned downstream of the converter. An indication of converter failure is provided when the sensor output exceeds a specified threshold value.
The inventors herein have recognized numerous problems and disadvantages with the above approach. For example, use of a hydrogen and/or carbon monoxide sensor appears to have the limitation of detecting converter degradation only when rich excursions in the engine air/fuel ratio occur and not when lean excursions occur. The inventors herein recognize that detection of lean excursions requires a nitrogen oxide sensor. Another problem of the above approach appears to be that transient operation under high engine load conditions may result in an erroneous indication of converter failure.